1. Field of the Invention
The present invention relates to a thermal type flow rate sensor disposed in fluid and arranged to generate heat when electric power is supplied so as to measure the quantity of heat deprived by the fluid, so that the flow rate of the fluid is measured.
2. Related Art Statement
In general, the quantity of sucked air into the engine is an important parameter about the operation state for an electronic-control internal combustion engine in order to control the basic fuel quantity and the basic ignition timing. Hitherto, a thermal type air flow meter having a resistor which depends upon the temperature has been put into practical use as an air flow meter for detecting the quantity of sucked air.
A conventional air flow meter is arranged to have a heat-generating resistor and a temperature compensating resistor, as sensors, supported in a branch passage formed in an air suction passage for dividing and passing a portion of sucked air. Furthermore, an equilibrium bridge circuit including the heat-generating resistor and the temperature compensating resistor is constituted, and the quantity of electric power to be supplied to the bridge circuit is controlled in a feedback manner so as to make the temperature of the heat-generating resistor to be higher than the temperature of the temperature compensating resistor by a predetermined degree. Therefore, the quantity of the electric current is adjusted so as to maintain the temperature of the heat-generating resistor even if the heat corresponding to the quantity of sucked flow is deprived from the heat-generating resistor.
As the resistors serving as the heat-generating resistor and the temperature compensating resistor, a resistor made of platinum wire as it is, a coil type resistor constituted by winding the platinum wire around a cylindrical bobbin and a resistor having an insulating substrate on which a platinum film is formed have been known.
In particular, as the aforesaid type resistor having the insulating substrate on which the platinum film is formed, the resistors disclosed in Japanese Patent Laid-Open No. 61-188901, Japanese Patent Laid-Open No. 60-236029, Japanese Patent Laid-Open No. 60-235020, Japanese Patent Laid-Open No. 2-269915, Japanese Patent Laid-Open No. 57-208412 and Japanese Utility Model Publication No. 3-34649 have been known.